Pneumatic sawtooth oscillator



United States Patent O 3,228,301 PNEUMATIC SAWTOGTH OSCILLATGR Victor W. Eolie, Tustin, Calif., assgnor to iowa State University Research Foundation, Inc., Ames, Iowa, a corporation Filed Feb. 27, 1963, Ser. No. 261,314 2 Claims. (Cl. 91-5) This invention relates to a pneumatic sawtooth oscillator, and, more particularly, to a pneumatic system capable of inating a balloon rapidly and deating it slowly so that the variation of the balloon volume with `time has a sawtooth waveform.

It is an object of this invention to provide a unique pneumatic system wherein a balloon is cyclically iniiated and deflated to provide an output characterized by a sawtooth waveform. Another object is to provide a system of the character described in .the object immediately preceding wherein the balloon is inated from Ian isolated tank of small volume charged to high pressure, -whereby danger of balloon bursting is avoided.

Still another object of the invention is to provide a pneumatic system capable of cyclically inating and deating -a balloon having only a negligible wall tension when inflated, in conjunction with a self-actuated fourway, two-position valve.

Yet another object is to provide a pneumatic system providing a sawtooth waveform output wherein ilow constriction means are provided for varying both the slope and the period of the waveform. Other objects and advantages of the invention may be seen in the details of construction and operation set down in this specification.

The invention is described in conjunction with the accompanying drawing, in which FIG. l is a schematic representation of the inventive system;

FIG. 2 is a schematic representation of a simplied version of the system for a mathematical derivation appearing hereinafter; and FIG. 3 is a graph of the variation of pressure (P).at the balloon port as a function of time (T).

Numerous occasions arise in pneumatic systems applications where it is necessary to design an oscillator having a sawtooth output waveform, i.e., driving pistons which must oscillate a load back and forth such as in an automatic production line. If rapid cycling or steep wave fronts are not necessary, the sawtooth waveform can be formed by means of mechanical-linkage feedback to ya conventional cam-stem four-way valve from the piston of a standard cylinder tted with a controlled orice to slow the return stroke. However, such la method gives a system too complex, heavy, noisy and slow for many applications.v In contrast to this, the rapid inflation-slow deflation means disclosed herein provides a system uniquely meeting the requirements of a high speed, simple, light- Weight system.

Such a system is seen in FIG. 1, wherein the numeral refers to -a four-way valve coupled to a pressure source (of say 90 p.s.i.g.) as at 11 and to a suction source as at 12. The valve 10 is equipped with a spool 13 (shown in chain line), biased downwardly by means of a spring schematically represented at 14, The spool 13 is adapted for two-position operation, with the ow passages in one position seen in solid line as at and 16, while those in the alternative position are `shown in dotted line as lat 17 and 18.

The other elements of the `sys-tem include a storage tank 19 and an inatable balloon 20. A rst conduit 21 icouples the valve 10 to the tank 19, the iirst conduit 21 being equipped wi-th a check valve 22. A second conduit 23 couples lthe valve 10 to the balloon 20 at the balloon port Patented Jan. 1l, 1966 r-r' i Ice 20a and is equipped with a flow constriction device gene-rally design-ated 24. The device 24 is made up of a check valve 25 and a throttle valve for variable constriction 26.

A third conduit 27 couples the rst conduit 21 to `the pilot end ofthe valve 10 as at 28 for overcoming the bias of spring 14. The third conduit 27 is equipped with a ow control device generally designated 29, made up of a check valve 30 and a throttle valve 31, similar to the arrangement designated 24.

The ow control devices 24 and 29 may be Modernair Valves No. 100, obtainable from Schrader Air Control Products Co., of New York city. Also, the valve 10 is obtainable from Schrader Air Control Products Co., being a four-way, lsingle pilot-operated, Itwo-position valve.

In FIG. 1, the spool 13 is in its lower position and, as such, connects the pressure source (as at 11) to `the tank 19 and via the third conduit 27 to the pilot port 28. This results in delivering pressure fluid to the tank 19 and relatively slowly deliver-ing pressure -uid to the pilot port 28. Depending upon the lbias in the spring 14, the volume of the piping system and the setting of the throttle valve 31, a time elapses until the pressure built up at pilot -port 28 is sufcient to shift the spool to its upper position. Thus the slope and period of the sawtooth waveform of FIG. 3 may be varied. In FIG. 3, the steeper slope line is seen to be dotted corresponding to the spool position coupling the tank with 4the balloon via the dotted passage 18. During the time of this pressure build-up in the pilot port 28, the balloon 20 is being deflated .through the throttle valve 26 by the suction source 12.

As soon as the -spool 13 has shifted lto the upper position, the tank 19 is in direct communication with the balloon 20 (via the check valve 25) to insure rapid ination of the balloon. Meanwhile, via passage 17, .the suction source 12 is in communication with the third conduit 27 Ito reduce the pressure at the pilot port 28, permitting the spring 14 to return the spool 13 to its lower position.

The device shown in FIGURE l overcomes the possibility of bursting, which is characteristic of any conventional constant pressure source for inating the balloon. Any such -source requires a valve which could accidentally stick open, and if this occurred even for a slight time, .the balloon would burst. Since it is quantity rather than pressure which is desired in the balloon, this can be avoided by inating the balloon from an isolated tank of small volume, charged to a relatively high pressure. FIG. 2 shows a sketch of the problem, and the mathematical derivation of the required tank volume in terms of charging pressure, atmospheric pressure, and the desired nal balloon volume is set forth below.

Derivation In FIG. 2, the portion of the system to the left of the valve D is designated 32, and this includes a rigid tank and pipe having a volume VT, an initial pressure 11T, and a final pressure p.

The valve D is initially closed, and to the right of the valve is provided a rigid pipe 33 which has a volume Vp and an initial pressure pA. The numeral 20 designates the balloon (shown in deflated condition) which has an initial volume of 0, `a nal volume of V, and a final pressure p.

In terms of absolute (gauge plus atmospheric) pressures, the following equation applies:

PTVT+PAVp=(VT+Vpl-V)P (1) It the balloon wall tension remains negligible yafter iniiation, the final pressure p is equal to atmospheric pressure pA, so that Equation (l) becomes:

VT- m From the foregoing, it is evident from the resultant formula that a high charging pressure is necessary not only to give a rapid ination through the valve, but also to avoid an excessively large tank volume.

For rapidly inating the balloon and slowly deating the same in a cyclic, sawtooth wave form manner, the valve '10 in FIG. 1 functions automatically to perform the following sequential operations:

(a) Charging the tank 1-9 from a high pressure source while the balloon 20 is being slowly deated by connection to low pressure suction;

(b) Isolating the tank from the high pressure source and the balloon from the low pressure suction;

l(c) Rapidly inflatiug the balloon from the charged tank; and

(d) Successively repeating the sequence of three events.

The relatively simple pilot-operated four-Way valve performs this 4automatic operation simply and reliably in that Iall ports are closed during the transition from one spool to the other. Oscillation frequencies of .0.1 to cycles per second are realizable with the design indicated.

The spool of the four-way valve will change position when the pressure at t-he pilot port 28 exceeds a certain threshold value Abased upon the bias of the spring 14. The delay time during which the spool 13 remains at its normal position is thus a function of .the spool threshold pressure, the adjustable ow resistance in the ow control device 29, and the small volume-trap for the air between the four-way valve spool 13 and the ow control device '29. As soon as the spool 13 of the four-way valve 10 changes position, further flow from .the high pressure source is blocked, the suction pressure is removed from the balloon and applied to the ow control device 2'9, the tank quickly inflates the balloon, and the four-way valve spool 13 returns to its normal position slightly later (due to spool inertia and drag).

The cycle time of the oscillation may be varied by adjustin-g the orifice in the ow control device 29 (as at 31). Very slow oscillations may be achieved by increasing the volume of air trapped between the four-Way valve spool 13 and the 110W control device 29. The quantity of air delivered to the balloon at the end of each cycle can be varied `by adjusting the volume of the tank (which may take the form of a cylinder with its adjustably-xed piston). VThe highest attainable oscillation frequency is a function of the minimum drag and inertia of the four- .Way valve control spool and the lowest practical flow resistance of the valve ports in connecting lines.

The balloon referred to in the specific description is used only as -an example, and may be replaced by any other expandable chamber such as a cylinder and piston or the pilot port of another pneumatic or hydraulic valve.

While in the foregoing specification a detailed description of an embodiment of the invention has been set down for the purpose of illustration, many variations in the details herein given may be made by those skilled 4in the art Without departing from the spirit and scope of the invention.

I claim:

1. A pneumatic sawtooth oscillator, comprising a fourway valve comprising a casing, a two-position spool lin said casing, spring means in said casing biasing said spool to one position, and port means located in said casing for pneumatically overcoming the bia-s of said spring means, a Ysource of pressure iiuid coupled to said valve, a pressure iluid tank, a irst conduit coupling said valve and tank for flow of pressure uid only from said source to said tank and only in said one position of said spool, a second conduit coupling said first conduit with said port means, a suction source also coupled to said valve, a portequipped balloon inatable by expanding pressure fluid from said tank, and -a third conduit coupling said valve and said balloon for flow of pressure uid from said tank to said balloon only in the -other position of said spool, said spool in said one position coupling said balloon to said suction source, and flow constriction means in said second and third conduits operative to regulate the flow therein when said spool is in said one position.

2. A pneumatic sawtooth oscillator, comprising a fourway valve comprising a casing, a two-position spool in said casing, spring means in sai-d casing biasing said spool to one position, and port means located in said casing for pneumatically overcoming .the bias of said spring rmeans, a source of pressure fluid coupled to said valve, a pressure uid tank, a rst conduit coupling said valve and tank for ow of pressure uid only from said source to said tank and only in said one position of said spool, a second conduit coupling said first conduit with said port means, expansible chamber means inflatable by expanded pressure uid from s-aid tank, a third conduit coupling said valve and expansible chamber means for ow of pressure fluid from said t-ank to said expansible chamber means only in the other position of said spool, a suction source also coupled to said valve, said spool in said one position coupling said expansible chamber means -to said suction source, and ow constriction means in said second and third conduits operative to Iregulate the ow therein when said spool is in said one position.

References Cited by the Examiner UNITED STATES PATENTS 840,876 1/ 1907 Steedman 91-443 1,861,742 6/1932 Hand 9ll-443 2,095,863 10/1937 Rockwell 60-60 y2,308,2'l'l l/i1943' Stegelitz 91-38 2,437,991 3/ 1948 Baston v. 91-38 FRED E. ENGELTHALER, Primary Examiner,

SAMUEL LEVINE, Examiner, 

1. A PNEUMATIC SAWTOOTH OSCILLATOR, COMPRISING A FOUR- 